Influence of lignin and cellulose from termite-processed biomass on biochar production and evaluation of chromium VI adsorption

The increasing water contamination by toxic heavy metals, particularly hexavalent chromium, has become a significant environmental concern. This study explores the pyrolysis of termite-processed biomass, specifically Pinus elliottii particleboard and its termite droppings (TDs), to produce biochar and its application for chromium (VI) adsorption. Termite droppings, rich in lignin, and particleboard, rich in cellulose, were pyrolyzed at various temperatures to assess the effect of biomass composition on biochar properties. The study found that lignin-rich termite droppings produced biochar with higher fixed carbon content and specific surface area than cellulose-rich particleboard biochar. FTIR and Raman spectroscopy revealed significant molecular structure changes during pyrolysis, which influenced the adsorption capabilities of the biochar. Adsorption experiments demonstrated that TD biochar exhibited significantly higher chromium (VI) adsorption capacity, attributed to its distinct chemical composition and enhanced surface properties due to higher lignin content. These findings underscore the crucial role of lignin in producing efficient biochar for heavy metal adsorption, highlighting the practical applicability of termite-processed biomass in water purification technologies.


Pyrolysis
Biomass pyrolysis in an inert (nitrogen) atmosphere was used to produce biochar from the milled original particleboard and termite droppings.The biochar was obtained using 20 g of the biomass in a horizontal tubular quartz furnace at temperatures of 450, 550, and 650 °C, with a heating rate of 10 °C min −1 , under nitrogen flow (250 cm 3 NPT min −1 ) to prevent the entry of O 2 and to remove the pyrolysis gases released during the process.The soak time for all experiments was 30 min.After pyrolysis, the biochar was collected from the reactor and saved for subsequent characterizations.

Thermal behavior
Thermogravimetric analysis (TGA) was used to determine termite-induced changes in the general characteristics of lignocellulose decomposition under pyrolytic conditions.TGA was conducted using Shimadzu TG-50 equipment.Approximately 5 mg sample was loaded into a platinum pan and heated from 25 to 700 °C at a heating rate of 10 °C min −1 under a nitrogen atmosphere with a flow rate of 50 mL min −1 .

Functional groups, composition, and morphology
The Fourier Infrared (FTIR) analysis (IRAffinity-1, Shimadzu) using the ATR method, with samples diluted in KBr, was used to evaluate the functional groups and their changes during the pyrolysis process.Raman spectroscopy (InVia, Renishaw) was employed to assess the carbonaceous characteristics of the samples.Scanning electron microscopy (SEM, EVOMA10, Carl Zeiss) was used to examine the morphology of the particleboard and termite droppings before and after the pyrolysis at different temperatures.N 2 adsorption/desorption isotherm analysis (Nova 1000e, Quantachrome) was performed to evaluate the porosity (by DFT method) and specific surface area (SSA, using the BET method).For this analysis, 0.1 g of each sample was dried at 100 °C for 24 h in an oven and 2 h at 300 °C in vacuum.

Determination of pH and pH pzc in biochar
The pH of biochar was determined using a method for testing the pH of waste materials (EN 12457-2:2002).The point of zero charge (pH pzc ) was used to characterize the surface acidity and alkalinity of the activated carbon 16 .

Adsorption isotherm
Adsorption isotherm models were employed to examine the relationship between the mass of the biochar used and the amount of chromium (VI) adsorbed at equilibrium, with chromium concentrations ranging from 40 to 100 mg L −1 .These models were also utilized to elucidate the interaction between chromium adsorbent molecules and surface adsorption sites.The optimal conditions maintained were an adsorbent dose of 0.5 g/100 mL, a solution pH of 5.5, and a contact time of 10 h under shaking.After that time, the biochar system reached equilibrium concentration.Then, the solutions were centrifuged, and the chromium (VI) concentrations were determined using a UV-vis spectrophotometer (Cary 7000, Agilent) at a wavelength of 540 nm, employing 1,5-diphenylcarbazide as an indicator.
The Langmuir and Freundlich isotherm models were applied to evaluate the fitness of the data and to determine whether the adsorbent surfaces were homogeneous or heterogeneous 16 .The Langmuir isotherm model assumes that adsorbates form a monolayer on binding sites.The Freundlich isotherm model assumes that the entire biochar surface is multilayered during adsorption.The equations used are presented in Table 1.
The adsorption capacities of the biochar were determined using Eq. ( 1): where q e (mg g −1 ) is the adsorption capacities of the biochar; C 0 (mg L −1 ) and C e (mg L −1 ) are the chromium (VI) concentrations before and after adsorption, respectively; V (L) is the volume of the adsorbate solution; M (g) is the amount of biochar.The adsorption kinetics was described by the pseudo-first-order (Eq.2) and pseudo-second-order (Eq. 3) models 17 : where q e (mg g −1 ) is the equilibrium capacity, q t (mg g −1 ) is the instant adsorption capacity, and k 1 (h −1 ) and k 2 (g•mg −1 •h −1 ) are the corresponding adsorption rate constants.

Biomass characterization and pyrolysis
Table 2 indicates the lignocellulosic composition of P. elliottii particleboard and its termite droppings.
The cellulose content of the biomass was reduced by 75% due to termite activity.These changes resulted in the TDs being richer in lignin, comprising 52.6% of its mass, doubling the relative amount of this compound compared to the original particleboard, which contained 25% of its mass.Ke et al. 18 also determined that the lignin content in termite droppings was concentrated compared with the undigested wood.Using chromatographic ( 1) (2) ln q e − q t = lnq e − k 1 t (3) q eq is the amount adsorbed by chromium (VI) at equilibrium, mg g −1 C eq is the the equilibrium concentration in solution, mg L −1 q max is the monolayer capacity of the adsorbent, mg g −1 K L is the Langmuir adsorption constant, L mg −1

Freundlich
ln q eq = ln(K F ) + 1 n ln C eq q eq is the amount adsorbed by biochar at equilibrium, mg g −1 C eq is the equilibrium concentration in solution, mg L −1 K F is the Freundlich constant, mg 1−1/n L 1/n g −1 1/n is the heterogeneity factor, - www.nature.com/scientificreports/techniques, they revealed that the relative number of lignin-derived components increased in the termite droppings.These findings support the idea that during the cell-wall degradation process and hydrolysis of cellulose by termites, the native lignin macromolecular assembly undergoes structural modification while conserving the interunit lignin linkage and retaining the original aromatic characteristics.Interestingly, there is a slight decrease in the hemicellulose content (approximately 4.5%).Hemicellulose has a more complex structure than cellulose, with abundant branched chains and substituents, which can be difficult for termite enzymes to digest.
Figure 1a, b present the morphology of the P. elliottii particleboard chips before and after the pyrolytic process at 650 °C, respectively.Figure 1c, d show the morphology of TDs in the same condition.
The particleboard chips appear elongated with varying lengths and diameters, exhibiting a plicated surface and irregular small pieces typical of knife-milled wood chips.The wood channel and wall structure were maintained after pyrolysis at 650 °C and are more evident after this treatment, likely due to the partial decomposition of the organic fraction.
The termite droppings also maintain their shape after pyrolysis.They are oval-shaped with six concave sides, exhibiting a smooth and homogeneous surface without evidence of large pores.Before the pyrolytic treatment, the average length of the TDs was 708.59 μm, and their width was 416.3 μm.The pyrolysis at 650 °C reduced the length and diameter by approximately 28%.
The TGA curves under the nitrogen atmosphere for both the particleboard and TDs are in Fig. 2. Considering that the TDs are rich in lignin, the comparative results indicate that the overall reaction rate decreases with the increase of the lignin content.
While comparing the thermal decomposition behavior of the TD to the particleboard, the TGA curve displayed fluctuations during the biomass decomposition.TD decomposition began earlier, and the maximum rate shifted to the left compared to the particleboard sample, indicating that a lower temperature can convert the TD sample to volatiles.The lowering of the initiation temperature corresponds to a decrease in the minimum energy required to start the active gasification reaction in the lignin-rich TD.Meanwhile, the maximum rate of thermal decomposition of the TDs decreased, indicating a lower amount of cellulose and/or hemicellulose 19 .
Initially, both materials exhibited mass loss of up to about 100 °C due to the loss of around 9% adsorbed water.The proximate analysis (Table 3) indicates that both samples have similar moisture content: 9.12% for the particleboard and 9.70% for the TDs.
Hemicellulose starts its decomposition easily, and the weight loss mainly occurs at 203-386 °C.Meanwhile, cellulose pyrolysis happens at a higher temperature range (286-426 °C), with the maximum weight loss at around 330 °C.Usually, cellulose has a small solid residue (below 4%) because most is volatilized at around 700 °C20 .Cellulose is depolymerized at around 350 °C, followed by further conversion via bond cracking and dehydration into levoglucosan (LGA), levoglucone (LGO), and other monosaccharides.The primary pyrolytic products from cellulose are anhydrosugars, which can be further converted at higher temperatures into light oxygenates (e.g., furans, aldehydes, ketones, acids, etc.) 21,22 .
The TGA data for the amount of hemicellulose and cellulose perfectly agree with the results from the Van Soest method (Table 2).In both analyses, the sum of these compounds in the particleboard was around 60%, while for the TDs, it was around 20%.
Among the three components, lignin is the most difficult to decompose compared to cellulose and hemicellulose, which exhibit higher weight loss.The behavior of the TD sample is typical of lignin-rich biomass in that the principal decomposition of lignin happened in a wide temperature range from around 300 to 550 °C, where it lost about 60% of its relative weight.
The TGA curves showed a larger residual amount of biomass in the TD sample at temperatures between 400 and 600 °C compared to the particleboard sample.A possible explanation is that once lignin is modified by chewing, part of its intermediate fragments will be rearranged through condensation and re-polymerization, leading to new structures with more stability.
The proximate analysis (Table 3) indicates that the ash content is 21% higher for TDs than the particleboard.This is because termites, through their feeding habits, often incorporate inorganic particles (dust, soil, and other minerals) into their droppings, resulting in a composite material that contains both organic and inorganic components.Additionally, with the consumption of cellulose and hemicellulose, the inorganics increase as they are not digested or adsorbed.The higher ash concentration could promote biochar yields, as inorganic elements in the ash are known to catalyze the formation of solid products during pyrolysis 23 .www.nature.com/scientificreports/ The TGA curve for the TDs, which is rich in lignin, evidences that the thermal mass loss is less pronounced than the particleboard curve.Lignin, being a highly cross-linked and three-dimensional polymer, can influence the pyrolysis process by increasing the heat resistance and decelerating the degradation rate of the biomass.Its intricate and heterogeneous chemical structure results in biochar with higher fixed carbon content and greater surface area than feedstocks with lower lignin content.The TDs, rich in lignin, exhibit a fixed carbon content 60% higher than the undigested particleboard and a higher surface area for the studied pyrolysis temperatures (Table 3).
Hemicellulose and cellulose, on the other hand, are simpler polymers that readily degrade during the pyrolysis process and can serve as a source of volatile organic compounds, presenting lower fixed carbon content.The P. elliottii particleboard, with 75% more cellulose than the TDs rich in lignin, had approximately 13% more volatile compounds than the TDs (Table 3).
Moreover, based on a comparison of volatile matter and fixed carbon, the yields of pyrolytic products can be predicted to be significantly different for the particleboard and TD biomasses.Considering the weights of the initial biomass and solid content after pyrolysis, the biochar yield indicates higher values, around 41%, for the lignin-rich TDs for each treatment temperature (Table 3).The particleboard, richer in hemicellulose and cellulose, contained the highest volatile matter (81.9%) and could be expected to yield the highest amounts of volatile products.After pyrolysis, the biochar obtained from particleboard (cellulose-rich) has around 24% of volatile matter remaining in the structure, indicating a yield of about 58 wt.% in volatiles.The lignin-rich TDs have a higher fixed carbon content (around 63% after pyrolysis) and can lead to higher biochar yields, as lignin has a complex structure and poses more resistance to thermal degradation than holocellulose (cellulose with hemicellulose), also due to its high level of aromaticity, size, and structural arrangement, which affects the proportion of the solid product generated 24 .Moreover, the decrease in biochar yield is due to increased pyrolysis temperature, probably from the thermal cracking of volatile components into lower molecular weight liquids and gases rather than biochar 25 .
Results obtained from FTIR spectra for the particleboard biochar demonstrated an increase in aromatic groups and a decline in acidic groups with increasing pyrolysis temperature (Fig. 3a).Biochar from particleboard pyrolyzed from 450 to 650 °C exhibited a decrease in relative intensity of the following bands as the temperature increased: 3500 cm −1 (O-H stretching of hydroxyl groups), 2900 cm −1 (C-H asymmetric and symmetric stretching of aliphatic groups), 1620 cm −1 (C=O stretching of carboxyl mode), and 1026 cm −1 (C-O symmetric stretching associated with cellulose, hemicellulose, and lignin).Other bands demonstrated an increase in relative intensity of around 900-800 cm −1 (C-H aromatic deformation modes), 1600 cm −1 (C=C aromatic stretching and C=O stretching of conjugated ketones and quinones), and 1400 cm −1 (C-C stretching of aromatic rings) 26 .
The termite-dropping FTIR spectra bands (Fig. 3b) were much more subtle than the particleboard ones.Up to 550 °C, few little alterations occurred in the intensity of functional groups.However, a more significant increase in the intensity was observed around 1600 cm −1 (C=C aromatic stretching and C=O stretching of conjugated ketones and quinones) with increasing pyrolysis temperatures.Since TDs were richer in lignin, a more complex biomass, the pyrolysis reaction was slower and occurred at a higher temperature.Consequently, the formation of byproducts took longer and required more energy.This slower reaction rate was likely the cause of the subtle changes observed in the FTIR spectra of this material.
The Raman spectra of the biochar obtained from particleboard chips (Fig. 3c) and TDs (Fig. 3d) at different pyrolysis temperatures helped elucidate the formation of carbon structures.For both types of biochar, the position of the D band (around 1400 cm −1 ) shifted toward a lower wavenumber for higher temperatures, while the position of the G band (around 1600 cm −1 ) remained constant, indicating the formation of larger aromatic ring clusters 27 .The I D /I G ratios increased, and the full width at half maximum (FWHMD) decreased with higher temperatures.This indicated greater structural order and the formation of larger aromatic ring clusters 19 .
In addition to the chemical composition, the porosity, including specific surface area (SSA) and micromesopore structures, are fundamental characteristics that significantly influence the efficacy of sorbents in the adsorption process.Both particleboard and TDs displayed greater SSA following the temperature increase (Table 3), attributed to the significant degree of organic matter decomposition associated with volatile release and the subsequent formation of a porous structure.Lignin is known to be more thermally stable than cellulose.Consequently, during pyrolysis, lignin tends to undergo decomposition over a broader range and at higher temperatures than cellulose (Fig. 2).As a result, the pyrolysis process for lignin-rich TD may involve more prolonged heating or higher temperatures to achieve decomposition, forming a more complex carbonaceous structure with a higher surface area and mesoporosity.Table 3 indicates that with the increase in SSA, the total pore diameter and the average pore diameter increased for both particleboards and TD samples.
These results have demonstrated that changes in lignin content have altered the surface physical properties of the biochar, which can positively influence its adsorption properties.

Adsorption isotherm
The parameters of the Langmuir and Freundlich isotherm models are listed in Table 4.
The data indicate that chromium (VI) adsorption onto particleboard and TD biochars fits both the Langmuir (R 2 values of 0.93-0.99)and Freundlich models (R 2 values of 0.82-0.97),suggesting the involvement of both chemical and physical adsorption mechanisms.Biochars pyrolyzed at 650 °C exhibited the highest adsorption capacities for chromium (VI), with maximum adsorption capacities calculated by the Langmuir isotherm being 71.0 mg g −1 and 51.0 mg g −1 for TD and particleboard biochars, respectively.
According to the Langmuir model, the dimensionless constant separation factor (R L ) can predict whether an adsorption system is favorable or unfavorable.The R L value indicates the type of isotherm: unfavorable (R L > 1), linear (R L = 1), favorable (0 < R L < 1), or irreversible (R L = 0).The R L values ranged from 0 to 1, indicating that adsorption of chromium (VI) on these biochars was favorable under the experimental conditions, demonstrating a high affinity for chromium (VI).

Adsorption kinetics
For the chromium (VI) adsorption capability of the biochar, the adsorption increased with the higher pyrolysis temperature for both types of biochar (Fig. 4).Initially, the absorption capability rose, but after approximately 24 h, a plateau was reached.The smooth and continuous nature of the curves indicates monolayer coverage of chromium (VI) on the surface of the biochar 28 .www.nature.com/scientificreports/ The best adsorption capacity for the particleboard chips was 100.66 mg g −1 ; for the TDs, it was 169.1 mg g −1 , both for samples heat-treated at 650 °C.These results can be directly associated with the SSA of the particles at different pyrolysis temperatures.For both particleboards and TDs, the SSA increased with the increase of the pyrolysis temperature (Table 3).This is likely due to the formation of more mesopores caused by the escape of volatile substances as the temperature increased 29,30 .According to Chen et al. (2012) 31 , a higher surface area and porosity of biochar are mainly associated with the decomposition of lignin, the rapid release of H 2 and CH 4 , and the reaction of aromatic condensation as the temperature rises.
The surface characteristics of biochar are crucial when considering its application as an adsorption agent in water systems.At lower pyrolysis temperatures, up to 650 °C, lignin is not entirely converted into hydrophobic polycyclic aromatic hydrocarbon (PAHs), making biochar more hydrophilic.At temperatures higher than 650 °C, biochar becomes thermally stable and more hydrophobic 32 .www.nature.com/scientificreports/Analysis of the FTIR spectra of both types of biochar (Fig. 3a, b) indicates that the aromatic groups are more pronounced in the particleboard samples.In contrast, the TDs do not show significant stretching bands for these compounds, possibly due to their lower content.This suggests that, in general, the TD samples can be considered more hydrophilic.Coupled with a higher surface area, this hydrophilicity resulted in a higher adsorption rate.
The main functional groups found in biochar that may contribute to the adsorption process include (i) aromatic rings; (ii) O-H groups, possibly from alcohols or phenols; (iii) C=O groups, possibly from carboxylic acids and esters; and (iv) C-H groups from aromatic and aliphatic compounds.These functional groups enhance the adsorption properties of biochar, making it effective in removing contaminants from water.
Independent of the temperature treatment, the pH of the biochar obtained from the particleboards was 6.0, and the pH of the TD biochar was 8.0.Pyrolysis studies indicate that higher temperatures lead to a progressive concentration of the inorganic constituents that comprise ash 33 , thus increasing the pH of the obtained biochar.The TD biochar exhibited a higher ash content at all pyrolysis temperatures.At 650 °C, the ash content of the TD biochar is 1.6 times that of particleboard biochar (Table 3).The pH pzc was utilized to assess the surface charge of the produced biochar.The TD biochar had a pH pzc of 7.2, and the particleboard biochar had a pH pzc of 6.0.Determining the point of zero charge in biochar is crucial for understanding its surface charge properties.At pHpzc, the net surface charge of the biochar is neutral, indicating an equal concentration of positive and negative charges on its surface.Above pH pzc , the biochar surface becomes negatively charged, attracting positively charged ions.
Conversely, below pH pzc , the surface carries a positive charge, thereby attracting negatively charged ions.For chromium (VI) adsorption, a solution of K 2 CrO 4 was employed at pH 5.5.Under these conditions, the potential adsorption ions are Cr 2 O 7 2− and, eventually, CrO 4 2− , which can consequently be attracted to the biochar surface.The results of pseudo-first-order and pseudo-second-order rate equations for the particleboard chips and TDs are presented in Fig. 4c, d and e, f, respectively, and Table 5.
The kinetic parameters at the equilibrium state q e and adsorption constant K 1 calculated from the pseudofirst-order model (PFOM) are indicated in Table 5. PFOM is based on the premise that the rate of solute uptake change over time is directly proportional to the difference between the saturation concentration and the amount of solute uptake by the solid over time.Adsorption kinetics frequently adhere to the Lagergren pseudo-first-order rate equation when adsorption occurs via diffusion through the interface.However, the outcomes obtained in this study are inconsistent with the experimental results and have low correlation coefficients (R 2 ), indicating that this model is unsuitable for chromium adsorption (VI).
The pseudo-second-order model (PSOM) assumes that chemisorption is the rate-determining factor for adsorption, estimating the behavior across the entire adsorption spectrum.Under these circumstances, the adsorption rate depends on the adsorption capacity rather than the adsorbate concentration.In this work, the correlation coefficients (R 2 ) of PSOM were higher (over 0.99) than those of PFOM, thus supporting that the adsorption of chromium (VI) by the obtained biochar was a chemisorption process.As described by the pseudosecond-order model, the adsorption kinetics seems reasonable since it takes longer to reach equilibrium, with the limiting step being chemisorption.This indicates that the rate is determined by the interaction with surface sites, which aligns well with the observation that the adsorption rate was higher for the higher SSA lignin-rich TD samples.

Hexavalent chromium adsorption mechanisms
Based on the results obtained, four adsorption mechanisms for chromium (VI) using biochar derived from P. elliottii and its termite droppings can be proposed: (i) Surface adsorption: the adsorption of hexavalent chromium from potassium chromate (K 2 CrO 4 ) solutions onto biochar primarily involves surface interactions between the chromium VI ions and the biochar.The biochar produced from termite-processed biomass and P. elliottii particleboard exhibits various functional groups (hydroxyl, carboxyl, and phenolic groups) crucial for adsorption 34 .Additionally, the large surface area of the biochar facilitates physical adsorption, enhancing the overall adsorption capacity.(ii) Electrostatic interaction: chromium VI exists as chromate ions (CrO 4 2− ) in aqueous solutions.These negatively charged ions can be attracted to the positively charged sites on the biochar surface, facilitating their adsorption 35 .(iii) Ion exchange: the functional groups on the biochar surface can exchange ions with the chromate ions 34 .

Figure 3 .
Figure 3. Results of the FTIR and Raman analysis of the (a,c) particleboard chips and (b,d) termite droppings (TDs) after the pyrolysis at different temperatures.

Table 1 .
Langmuir and Freundlich models of equilibrium adsorption and sorption capacity.

Table 2 .
Hemicellulose, cellulose, and lignin content of undigested particleboard and in the termite droppings.

Table 3 .
Specific surface area (SSA), total pore volume, average pore diameter, moisture, volatile matter, ash, fixed carbon content, and biochar yield of Pinus elliottii particleboard and termite-dropping samples.

Table 4 .
Parameters of the Langmuir and Freundlich isotherm models.

Table 5 .
Kinetic parameters of chromium (VI) adsorption on particleboard (PB) and the termite droppings (TDs) after pyrolysis at different temperatures.